In the aftermath of the averted war with Heaven, Alastor and Lucifer both have to adapt to the new normal. Alastor has his freedom but no idea what to do with it! Lilith is back and Lucifer finally has his closure: he's divorced. Yay?
The one constant for both of them is each other. They must face revelations about just what their rivalry means to them... and the desires that it inflames.
Or: Alastor falls first (and hard) but he drags Lucifer right down with him.